A modem-based echo cancellation enhancement for digital communications networks is provided. The effects on echo cancellation of non-linearity in a far end echo path in a digital telephone system are reduced by introduction of non-linearities into the near end signal prior to generation of the echo cancellation signal. Near end signal is sampled after linear to PCM conversion. A PCM to linear conversion is added prior to the echo canceler. The near end delay line can be provided between the converters to delay the signal as PCM samples, thereby reducing delay line memory requirements.
Legal claims defining the scope of protection, as filed with the USPTO.
1. A modem, comprising: a modem transmitter providing a transmit signal; a linear-to-PCM converter receiving said transmit signal and producing a PCM transmit signal; an echo canceler; a PCM to linear converter coupled between said linear to PCM converter and said echo canceler, providing linear transmit signal samples to said echo canceler; and a delay line, coupled between said linear to PCM converter and said echo canceler, for delaying said PCM transmit signal to match a network delay of a far-end network echo.
2. The modem of claim 1 , further comprising: a second PCM to linear converter configured to receive signals from an external source; a summer coupled to said second PCM to linear converter and to said echo canceler, wherein said second PCM to linear converter provides a receive signal to said summer and said echo canceler provides an echo cancellation signal to said summer, for combining said receive signal and said echo cancellation signal to provide an echo-reduced receive signal; and a modem receiver coupled to said summer to accept said echo-reduced receive signal.
3. The modem of claim 2 , wherein said received signal does not include a near-end echo.
4. The modem of claim 1 , further comprising: a delay line coupled between said linear to PCM converter and said PCM to linear converter to delay said PCM signal for delaying said PCM transmit signals to match said network delay of said far-end network echo before reconverting in said PCM to linear converter.
5. The modem of claim 4 , wherein said delay line comprises: storage for said PCM signal wherein said storage comprises storage adequate to hold delay line samples that match said far-end echo in said network.
6. The modem of claim 1 , wherein said delay line is coupled between said linear to PCM converter and said PCM to linear converter for transmitting said PCM transmit signals directly to said PCM to linear converter.
7. The modem of claim 1 , wherein said delay line is coupled between said PCM to linear converter and said echo canceler for transmitting said PCM transmit signals directly to said echo canceler.
8. A method for enhanced echo cancellation in a digital modem comprising: receiving modem transmit signals comprising a far-end network echo, converted from linear to PCM transmit signals in a linear to PCM converter; reconverting, in a PCM to linear converter, said PCM transmit signals to reconverted linear transmit signals; producing an echo cancellation signal in an echo canceler from said reconverted linear transmit signals; and delaying said converted signals in a delay line, prior to said producing, to match a network delay of said far-end network echo.
9. The method of claim 2 , further comprising: receiving PCM signals from an external source; converting said received signals from PCM to a linear receive signal; subtracting said echo cancellation signal from said converted linear receive signal to achieve an echo-reduced receive signal; and providing said echo-reduced receive signal to a modem receiver.
10. The method of claim 2 , wherein said received PCM signal does not include a near-end echo.
11. The method of claim 8 , wherein said delaying comprises delaying said PCM transmit signals to match a network delay of said far end network echo before said reconversion.
12. The method of claim 11 , wherein said delaying comprises transmitting said PCM transmit signals directly to said PCM to linear converter.
13. The method of claim 8 , wherein said delaying comprises delaying said PCM transmit signals, after said reconverting and before said producing, to match a network delay of said far-end network echo.
14. The method of claim 13 , wherein said delaying comprises transmitting said PCM transmit signals directly to said echo canceler.
Cooperative Patent Classification codes for this invention. Click any code to explore related patents in that topic.
December 27, 1999
September 21, 2004
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